Low-noise miniature electric motor

ABSTRACT

A low noise brushless dc motor has a rotor with a permanent magnet and a stator with a bearing system mounting a shaft for rotating the rotor. The bearing system includes at least one axially extending bearing surface that terminates in axially spaced apart ends defining the outer axial limits of the bearing surface. Magnetic components of the rotor and the stator define a magnetic circuit. the magnetic components have an eccentricity creating an asymmetry in the magnetic circuit that causes a resultant lateral magnetic force to be applied on the shaft at a point axially inside the outer axial limits of the bearing surface, whereby the shaft exerts a pressure along the bearing surface between both ends of the bearing surface in a direction perpendicular to the rotor axis.

This application is a continuation-in-part of U.S. Ser. No. 419,872 filed Oct. 11, 1989 now abandoned.

The present invention concerns an electric motor of the type covered by the wider concept according to claim 1.

A single coil motor of this type is known from disclosure DE OS 22 60 069 and a similar design has also been described in the utility document DE GM 87 02 271.

Motors of this type, which may be installed in road vehicles or office equipment, are generally enclosed in a sheet metal housing, which tends to produce mechanical and acoustic vibration. The latter should preferably be avoided, if possible, while at the same time low-cost manufacture remains a requirement.

An arrangement is already known from disclosure document DE OS 35 28 121 in which a lateral force is exerted on a single or multiple surface bushing, used to provide accurate guidance of a plain bearing, in order to generate a so-called secondary force to diminish rotational fluctuation in the space between the two bearing halves. However, this arrangement requires a level of complexity which does not appear justified for motors of this type in the context of the solution concerned in this case, and it would therefore be a disadvantage.

The disclosure DE OS 38 08 222 describes a particularly flat dc motor in which the asymmetric arrangement of the stator stirrup exerts a lateral force on the bearing as well, whereby its arrangement at the extremity however produces so-called edge running, i.e. the shaft is displaced on one edge in the bearing. A simple brushless motor with a flat air gap within the wider concept is known from disclosure DE OS 28 04 549. When a motor of this type is operated in a vertical plan without lateral bearing loads, it tends to produce a considerable amount of noise. The purpose of the present invention to reduce or eliminate this effect by the simplest possible means. Even though the remaining structure of this device does not tend to produce resonant vibration, the objective is to achieve very smooth running.

Thus, the purpose of the present invention is to provide a motor within the wider concept, of particularly simple construction, as characterized in this concept, whereby the production of noise and resonance is eliminated, or at least drastically reduced, in a design according to this wider concept which is cost-effective in itself and easy to assemble as a high volume product.

The solution to this problem is obtained by the provision of an asymmetric arrangement of the components of the magnetic circuit with respect to the rotor axis, and in particular, of the ferromagnetic components of the stator.

Further details and preferred further embodiment of the present invention are provided in the description which follows and in the drawings.

FIG. 1 is an enlarged cross section of a motor according to the present invention with a flat air gap;

FIG. 2 is a section at a line II--II in FIG. 1;

FIG. 3 is a section at a line III--III of FIG. 4 a further embodiment of a motor according to the present invention with a cylindrical air gap;

FIG. 4 is a section at IV--IV of FIG. 3, omitting the winding;

FIG. 5 is a first wiring diagram; and

FIG. 6 is a second wiring diagram for operation of a motor according to this invention.

FIG. 1 is an enlarged view of a miniature flat airgap electric motor with a permanent magnetic rotor 1. As shown in, a rotor magnet 2 may consist of 4 magnet segments. The rotor magnet 2 is fastened concentrically within a holder 3, which may be a plastic injection molding or a pressure die casting.

The holder 3 includes a soft magnetic disk 4 which bears on one face of the rotor magnet 2. The rotor magnet 2 is fastened to one face of the holder 3, while a fan wheel 5, which preferably takes the form of a molding cast in one piece together with the holder, for example, is placed at the other face of the holder 3. A shaft 6 is potted like the disk 4 and is thus fastened with the holder 3 so as to rotate together with it. However, the shaft 6 can also be fastened in the central bore in the holder 3 by means of a bonding agent, press fit or other means.

The stator 7 of the motor according to the present invention contains, inter alia a bearing carrier part 8. A central countersunk bore 16 is provided at the axially external face 10 of the bearing carrier part 8, and the diameter of this bore exceeds that of a bearing bore 17. The countersunk bore 16 is sealed by a bearing plate 18 whose external diameter is secured against axial displacement. The end of the shaft 6, which takes the form of a rounded tip 19 is brought to bear on the plate 18 under the effect of axial magnetic traction. A retaining ring 20 on the shaft 6 prevents the rotor from falling out. A housing shell 9 with a flat base 21 and an essentially cylindrical perimeter 22 is fastened to the outer casing of the bearing carrier 8. An iron core 11, potted in a plastic casing 23, bears on the base 21. A control magnet 24 is arranged opposite and radially around the outer perimeter of the face of the iron core 11 remote from the base 21, the control magnet also forms an integral part of the plastic casing 23.

An active or effective winding 12 and a disk of dynamo sheet metal are fastened in turn on the outer casing of the bearing carrier 8. Said disk consists of a D-shaped part 13 and a semi-circular locating ridge 14.

The outer diameter of the semi-circular D-shaped part is preferably no greater than the inner diameter 15 of the rotor magnet 2 and, this disk is positioned axially between the end of the winding head 12 and one face of the rotor magnet 2. The purpose of this arrangement is to allow the effect of the lateral force generated as a result of the eccentric configuration of the magnetic circuit to be applied axially at a point approximately mid-way between two bearing surfaces 30, 40 of the shaft 6. "Edge" running is prevented and the service life of the bearings is thus extended.

FIG. 2 shows a view along a line II--II in FIG. 1 of the configuration of the disk whose arrangement and shape are responsible for the asymmetry of the magnetic circuit. No further details of the motor are shown here because the present invention can be employed in motors of various designs.

In the example according to the present invention shown in FIGS. 1 and 2 a brushless dc motor is employed, with a permanent magnet rotor 1 and an active winding 12 providing motive power, which is energized in a single or dual pulse mode to generate an ac exciter field on the stator, and which, in conjunction with at least one soft magnetic and/or permanent magnetic component on the stator, creates one defined starting position for the rotor magnet as well as reluctance torque.

To eliminate disruptive jerky movement of the poles, the D-shaped part of the disk covers one pair of poles or a whole-number multiple thereof (up to a maximum of p-1).

In a bipolar motor (not shown), only one of the poles is preferably covered, whereby a corresponding degree of pole jerk must be accepted; however, even this arrangement provides lower running noise.

FIG. 3 shows a further embodiment of the present invention in the form of a slotted electronically commutated outer rotor motor with a cylindrical air gap. A rotor bell 33 encompasses the four segments of the rotor magnet 2. The shaft is firmly fastened at the center of the base 34 of the rotor bell 33. A stator stack 41, provided with stator laminations 42 whose eccentric outline can be seen in FIG. 4, is fastened on the bearing carrier part 8, whose configuration corresponds to that shown in FIG. 1. Such an eccentric configuration extends over the whole parimeter of the stator. The diameter of one of the two pairs of poles in this four-pole version is smaller than the other. This arrangement also results in a wider air gap 43 on one side and a narrower air gap 44 on the other. The eccentric configuration of the magnetic circuit provides a resultant lateral force which is applied between the bearing points, and the shaft consequently exerts a pressure on the bearing points 30, 40 in the same direction. The effective winding 12, which essentially fills the slots 45, has been omitted from FIG. 4. Said lateral force will also be generated even if only some of the stator laminations are arranged asymmetrically in the stator stack. Furthermore, a stator stack with symmetric laminations and an eccentric center bore, (not shown) can be used also to generate the lateral force referred to above in further detail.

The provision of suitable changes to the outer contour of the rotor, such as a steel disk arrangement, for example, will result in the exertion of the required lateral force on the bearing points.

In mechanically commutated (mainly bipolar) motors the exertion of a lateral force on the bearing points can be obtained by the provision of systematic eccentricity between the armature and the steel magnet.

FIG. 5 shows a circuit for operation of the motor according to this invention. The power is supplied through lines 70, 71. Part of this circuit is designed as a so-called current level circuit and consists of resistors 72, 73 and transistors 75, 76. The current flowing through resistor 72 is the same as the current flowing through resistor 73 when the voltage on a tachometer winding 52 is equal to 0. As long as the tachometer voltage is 0, the collector potential (transistor 76) is at approximately half operating voltage, i.e., transistor 75 is conducting. This assures that the motor will start. As soon as the motor is turning, a voltage is induced in tachometer winding 52. Transistor 76 and transistor 77 are thus switched on and off in proper phase so the required pulsating current can flow in stator winding 81.

A capacitor 79 and resistor 74 form an RC element that suppresses voltage interference and reduces the steepness of the current flanks so this yields a further improvement in running noise. A Zener diode 80 provides the high voltage protection and reverse polarity protection.

Capacitor 61 in FIG. 3 of West German application P 38 38 367.5 can be omitted because it has been found that in many applications high frequency voltage interference does not occur. By eliminating this capacitor 61, it is possible to increase expenditures elsewhere in the circuit (FIG. 5). Thus, for example, resistor 72 has been inserted between plus line 70 and transistor 75. Transistor 75 becomes a component that assumes the function of diode 53 in FIG. 3 of the application P 38 38 367.5 by means of a line 78 (short-circuit collector/base of transistor 75). By using the transistor 75 instead of diode 53, no cost is saved but a greater simplification is achieved. Three identical components (transistors 75, 76, 77) can be used. Transistor 76 is used to amplify the tachometer signal from tachometer winding 52.

The current for the transistor 75 is adjusted with resistor 72. It is advantageous for good startup properties of the motor (stator coil 21) that resistor 73 be designed smaller than resistor 72, e.g., about half the size of the latter.

The better startup properties of the motor are necessary especially under extreme conditions (e.g., temperatures from -40° C. and/or at a low voltage of about 5 V) which can occur with an automobile, for example, whose power supply is provided by a 12 volt battery.

FIG. 6 shows another operating circuit. The stator coil here consists of coils 81, which are preferably bifilar or double-wound and are designed as one coil. The two coils 81 are alternately switched on by a Hall-IC 90 by way of transistors 94 and 95. Capacitors 93 and 97 assure a shallow increase in current and drop in current and when the operating circuit is designed as an IC. IC-internal resistors fulfill this function. This results in a reduction in noise in running and prevents voltage interference. Zener diodes 92, 96 and 98 together with a series resistor 91 and winding resistors provide reverse polarity protection and high voltage strength. 

I claim:
 1. A low noise brushless dc motor comprisinga rotor having a permanent magnet with an inside diameter; a shaft made fast in the rotor to rotate therewith; a stator; bearing means contained in the stator mounting the shaft for rotating the rotor and defining an axis of the motor around which the rotor rotates, the bearing means including at least one axially extending bearing surface rotatably supporting the shaft and terminating in axially spaced apart ends defining outer axial limits of the bearing surface; an iron core mounted on the bearing means and forming part of the stator; an active winding on the iron core providing an electromagnet in the stator when the winding is energized, magnetic components of the rotor and the stator defining a magnetic circuit in the motor, the magnetic components of the rotor and the stator having an eccentric configuration creating an asymmetry in the magnetic circuit and causing a resultant lateral magnetic force to be applied on the shaft at a point axially inside the outer axial limits between bearing points of the bearing surface, the shaft consequently exerting a pressure along the bearing surface between both ends of the bearing surface in a direction perpendicular to the rotor axis, whereby noise is reduced and service life of the bearing surface is increased; and magnet means mounted on the stator creating a defined starting position of the permanent magnet of the rotor and generating reluctance torque in the motor.
 2. A dc motor according to claim 1, wherein the eccentric configuration is formed in the stator and extends over whole perimeter of the stator.
 3. A dc motor according to claim 1, wherein the motor is of flat air gap construction and the lateral magnetic force applied on the shaft is generated by means of a D-shaped disk made of dynamo sheet metal disposed on the motor axis, the D-shaped disk being non-symmetrical with respect to the motor axis, whereby a greater portion of the D-shaped disk lies on one side of the motor axis opposite a lesser portion of the disk, and the D-shaped disk being disposed axially between the rotor magnet and the active winding.
 4. A dc motor according to claim 3, wherein the greater portion of the D-shaped disk extends over one of poles of the permanent magnet of the rotor in a bipolar motor, and extends over at least one pair of poles in a multipolar motor.
 5. A dc motor according to claim 4, wherein outer diameter of the greater portion of the D-shaped disk is no greater than the inside diameter of the permanent magnet of the rotor.
 6. A dc motor according to claim 1 wherein the motor is of cylindrical air gap construction and the stator comprises a stack of stator laminations, and wherein at least part of the stack has laminations whose outer contour is of eccentric configuration.
 7. A motor according to claim 6, wherein entire stator stack has laminations whose outer contour is of eccentric configuration. 